Prolactin is a peptide hormone from the anterior pituitary gland that participates in the regulation of a variety of targets. Its modus operandi is the subject of this research. The prevailing concept in this area which had previously guided the formulation of questions, and thus experimental design, was that peptide hormones, as "first messengers", cannot enter their target cells and, therefore, that the generation of "second messengers" through an interaction of the hormone with the cell boundary must be operative. In 1976, we demonstrated unequivocally not only that prolactin can enter its major target, the milk secretory cell, but that it does so physiologically. Furthermore, during its sojourn in the cell, it becomes intimately associated with the cell nucleus. Thus, an alternative to the "second messenger" hypothesis of peptide hormone action was born. The major objective of our present work is to continue to examine this alternative, namely, that prolactin regulation requires its direct contact with subcellular constituents. Our efforts during this particular period of support are directed toward an analysis of the iter- and intracellular distribution patterns of enzymatic and secretory protein markers of hormone action in relation to distribution patterns of endogenous, i.e., the animal's own, prolactin. This is being done largely in rat mammary tissue with both light and electron immunochemical microscopy. The significance of this research is that it constitutes a unique approach to resolving the most fundamental problem in biology -- the regulation of genetic expression.